The effective application of stereovision or multivision systems depends on the ability to generate and transmit (over a useful distance) synchronized stereovideo or multivideo signals from two or more cameras respectively. This problem is non-trivial and there have been numerous attempts to address it, as evidenced by the several related patents issued: U.S. Pat. Nos. 5,307,168, 5,903,308, 5,995,140, 6,340,991, 7,411,604, 6,950,121 and 7,511,764; Journal papers published: Lamport-1978, Lin-2005; Conference paper presented: Luhmann-2005 and finally a Princeton 2007 PhD dissertation by Chang Hong Lin, which are all hereby incorporated by reference.
Tashiro of Sony Electronics, Inc. describes in U.S. Pat. No. 5,307,168 an electro-mechanical means of synchronously controlling the mechanical shutter of two cameras. This is no longer applicable because today, most cameras on the market are digital cameras and in turn most of these use electronic shutters to sample the frames in a video sequence. An electro-mechanical synchronizing device would in any case be too slow for the multivision systems used in high speed applications.
Chen et al. of AT&T Corporation describe in U.S. Pat. No. 6,340,991 a way of using the motion of objects captured in video sequences by multiple cameras to calculate and compensate for the relative lag between the said video sequences. This avoids the need for explicit synchronization and shifts the problem to post processing. While this method is touted as a means of reducing costs, there are a number of scenarios where the net complexity and cost is increased by the need of an additional post processing step. Moreover, this technique is not universally applicable such as in cases where there is no motion in the captured sequences. This method of synchronization is additionally limited in the accuracy it can achieve since the resulting video sequences could still be misaligned by as much as half the inter-frame duration, on average.
Trinkel at al. of Deutsche Telekom AG describe in U.S. Pat. No. 7,411,604 a more general way of using cues artificially inserted into the recorded video sequences to measure and compensate for the relative lag between the said video sequences. In this case a modulated light signal (or audio signal) present in the field of view of both cameras is used provide a kind of time-stamp in the video sequences recorded by each. Although this method avoids the explicit need for motion in the captured sequences, it shares the remaining disadvantages of the method put forward by Chen et al. of AT&T, as described previously.
Cooper et al. of Ultrak, Inc. describe in U.S. Pat. No. 5,995,140 a system and a method which uses a pair of external synchronization signals produced by a video controller to directly control the vertical and horizontal synchronization between the cameras in a multivision system. However, in this system each camera possesses an independent oscillator to generate its own pixel clock. These oscillators are unlikely to operate in synchronism due to the absence of any mechanism that holds them in lockstep. The vertical and horizontal synchronizing signals are far too coarse to guarantee accurate pixel synchronization. Thus, while line and frame capture could be generally synchronized, the same cannot be affirmed at pixel level. This makes the system inadequate for synchronous digital transmission which requires perfect synchronization at every level.
Cooper at al. of Ultrak, Inc. have later described in U.S. Pat. No. 7,511,764 a method for generating synchronized digital video signals from a number of slave cameras connected to a master base unit. Although this system meets strict timing requirements, it does so in an unnecessarily complex way that makes it costly to implement. The slave cameras each posses their own oscillator and so does the master base unit. This, in turn, necessitates that the base unit adopts a store and forward mechanism to resynchronize the frames generated by the slave cameras. The base unit therefore requires a memory to temporarily store video data from each camera. This, not only adds complexity and cost, but also introduces a small but distinct delay in delivering the video data which may be a significant disadvantage for certain high speed applications.
In the system disclosed by Tserkovnyuk et al. in U.S. Pat. No. 6,950,121, a shared clock is used to guarantee electrical synchronization between a stereovision pair of analogue cameras. However, frame synchronization is only achieved by having one camera supply the other camera with a frame reset pulse. This defeats much of the advantage of using a shared clock since the delay incurred in transferring this signal from one camera to the next would result in a small but distinct difference in the timing behaviour of the two cameras. This problem grows as the distance between the cameras increases thereby posing serious limitations in the relative positioning of the cameras. In high speed video capture systems, this could impair the level of synchronization making it unfit for digital transmission.
Cooper et al. of Ultrak, Inc. describe in U.S. Pat. No. 5,903,308 a method that compensates for the delays caused by the transfer of synchronization pulses from one camera to the next. Although this method could offer a solution for cameras synchronized by such transfer of pulses, it would still be undesirable from the point of view of cost and complexity.
Lacking the knowledge of a simpler method for high synchronization, German vision experts Luhmann and Raguse from the IAPG and Volkswagen AG respectively went to the great length of using beam splitting optics (used in reverse) in order to guarantee the synchronicity of the multiple video sequences they wanted to capture-by using a single camera. This technique would by necessity, produce perfect synchronization between the two halves of the stereovision field of view, however, only at an exceptionally great expense. It is also exceedingly cumbersome and hardly applicable to the vast majority of applications which require a lightweight and portable stereovision or multivision implementation.
In 1978, Lamport analysed the problem of maintaining approximate clock synchronisation between distributed systems across a network. Lin in 2005 extended the concept to multiple distributed cameras and went on to write a PhD dissertation about it. However, all of these methods rely on the transfer of datagrams across a network and the synchronisation achievable is, of the order of tens of milliseconds and is also non-deterministic. For high speed applications this level of uncertainty would be unacceptable.
The foregoing description of prior art makes it amply clear that prior to the present invention, nobody, (whether ordinarily or extraordinarily), skilled in the art has been able to find an equivalent, simple, low-cost method for generating and transmitting precisely synchronized video from stereovision or multivision video sources such as camera systems. It follows, therefore, that the present invention is novel and non-obvious to anybody ordinarily skilled in the art.